Abstract
Deforestation in Amazon is expected to decrease evapotranspiration (ET)
and to increase soil moisture and river discharge under prevailing
energy-limited conditions. The magnitude and sign of the response
of ET to deforestation depend both on the magnitude and regional patterns
of land-cover change (LCC), as well as on climate change and CO2
levels. On the one hand, elevated CO2 decreases leaf-scale transpiration, but this effect could be offset by increased foliar area density.
Using three regional LCC scenarios specifically established for the
Brazilian and Bolivian Amazon, we investigate the impacts of climate
change and deforestation on the surface hydrology of the Amazon Basin
for this century, taking 2009 as a reference. For each LCC scenario,
three land surface models (LSMs), LPJmL-DGVM, INLAND-DGVM and ORCHIDEE,
are forced by bias-corrected climate simulated by three general circulation
models (GCMs) of the IPCC 4th Assessment Report (AR4).
On average, over the Amazon Basin with no deforestation, the
GCM results indicate a temperature increase of 3.3 °C by 2100
which drives up the evaporative demand, whereby precipitation
increases by 8.5 %, with a large uncertainty across GCMs. In the case
of no deforestation, we found that ET and runoff increase by 5.0 and
14 %, respectively. However, in south-east Amazonia, precipitation
decreases by 10 % at the end of the dry season and the three LSMs
produce a 6 % decrease of ET, which is less than precipitation, so
that runoff decreases by 22 %. For instance, the minimum river discharge
of the Rio Tapajós is reduced by 31 % in 2100. To study the additional
effect of deforestation, we prescribed to the LSMs three contrasted
LCC scenarios, with a forest decline going from 7 to 34 % over this
century. All three scenarios partly offset the climate-induced increase
of ET, and runoff increases over the entire Amazon. In the south-east,
however, deforestation amplifies the decrease of ET at the end of
dry season, leading to a large increase of runoff (up to +27 % in
the extreme deforestation case), offsetting the negative effect of
climate change, thus balancing the decrease of low flows in the Rio
Tapajós. These projections are associated with large uncertainties,
which we attribute separately to the differences in LSMs, GCMs and
to the uncertain range of deforestation. At the subcatchment scale,
the uncertainty range on ET changes is shown to first depend on GCMs,
while the uncertainty of runoff projections is predominantly induced
by LSM structural differences. By contrast, we found that the uncertainty
in both ET and runoff changes attributable to uncertain future deforestation
is low.
Citation
ID:
227478
Ref Key:
guimberteau2017hydrologyimpacts